The present invention concerns antennas of the “loop” type, used as transmission/reception beacons, particularly in a radio frequency identification system, by means of electronic rags. This invention more particularly applies to antennas which can be used in a system for detecting the passage of persons, animals, or objects of considerable size, each of which bears electronic tags which enable their identification at the moment of detection of said passage. Accordingly, a specific application of the invention is the automatic identification and detection of participants in sports events, especially in order to establish, in an automatic, instantaneous and certain manner, the timing and ranking of the participants in a sports event taking place on a predetermined course involving passage through intermediate points and/or over a predetermined starting line and/or finish line, especially trials in which many competitors are involved and the individual identification thereof by means of the usual, very empirical processes is consequently difficult.
Loop antennas are most often used as receiver antennas in order to capture an electrical and/or magnetic field, in association with a receiver-amplifier of the signal received.
These antennas are also used as transmitter antennas in order to create a magnetic field in certain highly specific applications. Indeed, at a short distance from a current loop, only the magnetic field remains significant, because the electrical field is very weak.
It should be remembered that such a loop antenna, used in order to create a magnetic field, consists of a simple wire which is an electrical conductor, looped around one or more turns, through which runs a current of constant intensity and phase along the entire length of the wire. This antenna may have the configuration of a circular, square, or rectangular loop.
In the case of a rectangular loop antenna, the magnetic field H on an axis perpendicular to the plane of the loop and passing through the center of the loop is given by the following formula:
                    H        =                  n          ⁢                                          ⁢                      I            ·                          ab                              p                ⁢                                                                            a                      2                                        +                                          b                      2                                        +                                          h                      2                                                                                            ·                          [                                                1                                                            a                      2                                        +                                          h                      2                                                                      +                                  1                                                            b                      2                                        +                                          h                      2                                                                                  ]                                                          [        FORMULA        ]            wherein:                n is the number of antenna wire loops,        I is the intensity of the current in that wire,        2a is the width of the rectangle formed by the loop,        2b is the length of that rectangle,        h is the difference from the point in question to the center of said rectangle.        
When the distance h becomes large relative to a, or a and b, the previous formula is simplified and becomes:
                              H          =                      n            ⁢                                                  ⁢                          I              ·                                                2                  ⁢                                                                          ⁢                  ab                                                  p                  ⁢                                                                          ⁢                                      h                    3                                                                                      ⁢                                  ⁢        or                            [        EQUATION        ]                                H        =                  n          ⁢                                          ⁢                      I            ·                          S                              2                ⁢                p                ⁢                                                                  ⁢                                  h                  3                                                                                        [        EQUATION        ]            wherein S designates the surface area of the loop (equal to 4ab).
We now see that the magnetic field is proportional to the total current n I passing through the loop and to the surface area S of the loop, and that this magnetic field H is inversely proportional to h3 and therefore decreases very quickly when the distance h increases—that is, when one moves away from the plane of the loop, while remaining on the central axis thereof.
It should also be noted that the frequency f of the current running through the loop does not directly intervene in the preceding formulas. In fact, this frequency intervenes due to the fact that the length of the loop wire must be small relative to the wavelength λ. In general, this wavelength is selected so as to be less than λ/4, thus ensuring that the current remains constant.
This type of loop antenna is most frequently implemented by associating the looped wire, of a given length, with an inductance L and a resistance R, to which is associated a capacitance C in such a way as to create a resonant circuit of the RLC type, with LCω2=1 (ωbeing equal to 2πf), so that the intensity I of the current within the wire is maximal, because it is only connected to the resistance R of the circuit. This resistance R is itself composed of the resistance of the wire itself, connection resistances, and an additional resistance which enables modulation of the overvoltage coefficient Q=Lω/R, but it then dissipates a non-negligible power R (n I)2.
Another factor which limits the implementation of this type of loop antenna is the actual capacitance of the wire that forms the loop. Indeed, when the length of the wire is increased, its actual capacity becomes considerable, approaching the actual resonance of the wire, which is the acceptable limit for the resonant structure.
In this way, the loop structure of a present-day antenna that transmits a magnetic field is characterized by small dimensions, taking into account the limiting factors set forth above. This structure thus remains rather sensitive to the outside environment because, given that its principal is that of a resonant circuit, numerous types of interference are liable to have an effect on the circuit itself. These include the effect of temperature—for example, as a result of the heating of the additional resistance; the effect of a metallic element placed close to the loop; the effect exerted by components themselves, such as capacitors, which may be the source of high voltages that vary with temperature.
As a result, the applications of present-day loop antennas, in practice, are limited to very local use. In other words, they create a significant magnetic field in a volume whose base does not exceed the surface of the loop and whose height ranges from a few tens of centimeters to a maximum of about 1 m.
The demand for the field of radio frequency identification, mentioned in the introduction to this document—that is, the detection and identification of the bearers of an electronic tag known as a “smart” tag, which responds to a query from a fixed beacon when the bearer passes close to said beacon—is greatest in connection with systems of considerable volume, in which the electronic tags can “dialogue” with the beacon.
In reference to FIG. 1 of the schematic diagram attached hereto, we see how such a radio frequency identification system is configured and operates. The fixed device includes:                a transmission/reception beacon 2 with a loop antenna 3;        a reader 4, which is connected by means of a coaxial cable 5 to the beacon 2 and which generates the traffic frequency modulated for the transmission of the command code (the frequency, in this case, being a standardized value of 13.560 MHz),        a computer 6 which controls the system via software programs for the detection, acquisition and management of the data conveyed to it.        
When an electronic tag 7 is located in the reading zone—that is, within a certain volume V indicated by dashed lines—said tag receives electrical power from the electromagnetic energy generated by the transmission/reception beacon 2. The frequency generated by this beacon 2 is modulated in amplitude by the electronic tag 7, according to a code specific to this tag 7. The modulated frequency is received by the reader 4 via the transmission/reception beacon 2.
In the case of a classic loop antenna 3, the beacon 2 is generally located vertically on one end of the volume V, as shown in FIG. 1. As is indicated by the preceding text, the volume V thus controlled by a present-day loop antenna remains quite limited, especially in its width L, but also in its height h and its depth P.
Nonetheless, when the system is applied to the detection of the passage of persons, animals, or objects of considerable size, the dimensions L, h and P may, in practice, have the following values:                the width L of the passage may be several meters, and even as much as 10 m or more (for example, the entire width of a street);        the height h is typically 2.5 m;        the depth P is related to the speed of displacement of the persons, animals, or objects and may reach several meters.        
Accordingly, a particularly large volume V is required for the use intended here, and classic loop antennas, such as those used by present-day transmission/reception beacons, cannot meet this need, as was explained above.